Recently, severe root rot and mortality was observed on young potted pistachio rootstock trees housed in a research plot in California. Dr. Elizabeth Fichtner, UCCE Tulare County and Dr. Cheryl Blomquist, California Department of Food and Agriculture found a new Oomycete, Phytopythium helicoides associated with the observed root rot and mortality. Phytopythium is a newly designated genus exhibiting characteristics similar to both Phytophthora and Pythium and is considered to be an evolutionary intermediary between the two genera. In collaboration with Dr. Greg Browne, USDA-ARS, and Dr. Blomquist, studies are underway in a greenhouse at the Lindcove Research and Extension Center to determine pathogenicity of P. helicoides to UCB1 pistachio rootstock.